A soft jaw lathe chuck generally comprises a plurality of jaw members movable reciprocally towards and away from a central point coinciding with the lathe axis. Each of the jaws includes a clamping member of relatively soft metal which can be drilled or otherwise machined to form an aperture of the desired size and shape to accommodate a particular type of workpiece. A typical arrangement comprises three reciprocally movable jaw members arranged at 120.degree. intervals around the axis and movable on threaded shafts or the like towards and away from the axis in directions perpendicular to the axis. On each of the jaw members is mounted an indexible hexagonal clamping member, these clamping members being so aligned that respective corners of each of them coincide on the lathe axis when the jaw members are moved inwards to the fullest extent. A hole can then be machined, along the lathe axis and centered on the meeting point of the three corners.
To enable a plurality of workpieces of different size and shape to be accommodated by a single assembly of jaws and clamping members, the clamping members are generally made symmetrical and indexible. In this way, up to six different sizes or shapes of workpiece can be accommodated by indexing each of the clamping members through 60.degree. at a time.
It's obviously important that the clamping members must be held securely in place during both the formation of the jaw aperture and during turning of a workpiece held in the jaw. To ensure this, clamping members which have been used hitherto are fastened to the jaw member by means of a central clamping screw and furthermore have a shallow recess on their undersides tapering inwardly towards the bottom surface of the insert to engage correspondingly shaped forward projection on the jaw member to which the clamping member is secured. This helps to prevent the clamping member from lifting under stress, but there remains the problem of accurate indexing of the clamping member and securing it in such a way that it will not twist laterally out of line during machining of the jaw aperture or during turning.
One arrangement which has been used hitherto for indexing the clamping members has been the provision of a longitudinal tenon on the upper surface of the jaw member to which the clamping member is attached, engaging a corresponding slot within the recess on the underside of the clamping member. The accurate machining on the underside of the clamping member first of a recess with inwardly tapering surfaces and then of three (in the case of a hexagonal clamping member) transversely extending slots in the inner surface of the recess is a difficult operation, and even if it can be done accurately the indexing of the clamping member is a complicated operation. The present invention aims to provide a simplified indexing and clamping arrangement which is easy to machine and easy to use.